Aluminosilicate zeolites are crystalline microporous solids with diverse framework structures and void networks constructed by arrangements of SiO44− and AlO45− tetrahedral units. These materials are widely used in adsorption, catalysis, and ion-exchange processes. Zeolites are typically synthesized by hydrothermal treatment of amorphous aluminosilicate gels in the presence of inorganic (e.g. Na+, K+, etc.) or organic structure-directing agents (OSDA) in hydroxide or fluoride media. OSDA reagents, in particular, increase the cost and the environmental burden of many large-scale zeolite syntheses.
Much effort has been devoted to the development of OSDA-free synthesis protocols to decrease such costs and the emission of toxic species in the gaseous and water streams generated during the synthesis or the subsequent treatments required to decompose organic species contained within zeolite voids. Recently, several groups have reported improved protocols for seed-assisted hydrothermal synthesis of zeolites from amorphous aluminosilicate gels without the use of OSDA species. These methods use large concentrations of alkali cations to stabilize the target frameworks and, as a result, have succeeded mostly in the synthesis of Al-rich frameworks (Si/Al<10). Similar protocols remain unavailable for OSDA-free synthesis of target zeolites (e.g., CHA, STF, MTW, MFI etc.) with lower Al contents, which are often preferred because of their greater structural and acid site stability. In some instances, it is simply not possible to grow a given zeolite structure of interest (e.g., STF, MTW, etc.) at conditions with a Si/Al of less than 10, or even 7.
Zeolites are kinetically (but not thermodynamically) stable towards conversion to denser framework structures (e.g. α-quartz). As a result, their synthesis often involves the formation of structures of intermediate stability in the course of forming the ultimate target structures, which are often rendered stable only by the use of specific organic or inorganic cations. Transformations of one zeolite structure into another one—interzeolite transformations—have been explored because they can provide a strategy for the selective synthesis of specific structures, often with shorter synthesis times. The mechanistic details of such interzeolite transformations, however, remain unclear and predictions of their success largely empirical.
Most reported interconversions use OSDA moieties to induce the nucleation of frameworks that are in fact of lower framework densities and thus less stable than the parent zeolite, or to form structures that would not form at all without the presence of an OSDA. Several studies have used seeds to assist the formation of desired structures without the aid of OSDA species; others have induced interzeolite transformations in the presence of both seeds and OSDA. Successful interzeolite transformations without either seeds or OSDA have been reported only for zeolites with low Si/Al ratios (Si/Al ratio of less than 10, generally from 2-5). To date, target materials with higher Si/Al ratios (Si/Al>10) do not appear to have been synthesized via interzeolite transformations without the aid of OSDA species.
Providing a more facile and cost effective method for synthesizing high silica zeolites would be of great value to the catalysis industry.